So 3 days ago I added a filter AND 3 new cories to gabe's 10 gallon planted tank. Today i noticed that his tail was torn up :( Do you guys think that the addition of the cories and the filter stressed him out and made him bite his tail? He still acts very much himself and he doesnt even pay attention to the cories so I really doubt the cories did it. The filter is baffled with a sponge. Could it be the filter? Could his tail have gotten stuck in the inflow tube?

Housing
What size is your tank? 10 Gal
What temperature is your tank? 80*F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? 3 cories + Live plants

Food
What type of food do you feed your betta fish? Hikari biogold + tropical flakes/bloodworms for variety
How often do you feed your betta fish? once a day

Maintenance
How often do you perform a water change? once a week
What percentage of the water do you change when you perform a water change? 50%
What type of additives do you add to the water when you perform a water change? Aquasafe declorinator + API iron fertilizer for plants

Water Parameters:
Have you tested your water? If so, what are the following parameters? Unknown

Symptoms and Treatment
How has your betta fish's appearance changed? torn tail fin
How has your betta fish's behavior changed? None at all
When did you start noticing the symptoms? Today
Have you started treating your fish? If so, how? No.
Does your fish have any history of being ill? No
How old is your fish (approximately)? Got him about 1.5 months ago

Besides clean water is there any way to go around treating this? I can move him to his old container and use it as a hospital tank if I must, but im afraid the temp at night will get too cold for him. Without the heater the temp at night will be in the low 70's. I cant add salt to the tank either as cories and plants are supposed to be sensitive to salt. What do I do? HALP!

Wait, I'm a little confused....so you JUST added a brand new filter....? How long has this tank been set up without a filter?
I'd also like to point out that Corys are VERY sensitive...if this is a brand new filter and your just starting your cycle, test your water daily and preform water changes whenever you see the slightest hint of ammonia...though its REALLY risky cycling with Corys....

But, back to the fish....sometimes Bettas nip their fins for the oddest reasons, especially HMs with their huge, elaborate finnage. It looks to me like he blew his fins, either from flaring or the flow from the filter, and then nipped at them a bit. Since you can't add AQ salt to the tank because of the corys, I would suggest heading out and picking up a small Kitter Kepper and a small heater and prepare him a little hospital tank with water changes every other day.

Oops. I clean the tank 2x a week. My mistake. Yeah My tank had no filter just the betta for a few weeks and I was doing 50%WC 2x a week with a gravel vac. I will continue to do that for a few more weeks. Hopefully by then the tank will cycled and I can drop down to 1x a week. I have spent enough money on this tank and really cant afford to be buying expensive water testing kits. Classes start in a few days and I need to buy textbooks.
I have a hard time believing cories are THAT sensitive. A few years ago when we didnt know anything about fish we set up a 10 gal tank and added a bunch of fish the same day we bought the tank. We cleaned it maybe once every other month and fed the fish only flake food. more than 2 years later the only original fish that were still alive were the 2 cories...
I have a bunch of ziplock plastic bins in the garage. They are opaque so the fish wont be able to see anything through it. I may clip one on to the side of the tank and put the betta in there. Thanks guys. can believe I didnt figure that out myself.

Some pet stores might test your water for free if you can't afford a test kit...though that's a lot of trips. Probably best to stick with small partial water changes as frequently as you can, more the 2x a week I'd suggest, at least 3x a week, and take the water in to be tested as often as you can....won't know if its cycled until you can get it tested xD

I've heard of the occasional Cory being pretty hardy..I think it depends on the species and such, personally....but for the most part they're very sensitive to water quality, and really shouldn't be added to new tanks. Just watch them though and keep up with water changes...hopefully they'll be okay..

Oh!! Ziplock containers are the best, love those things...yeah, you could float one of those in the tank, maybe add part of a plant or two in there for him...probably want to change the water in the container every day-every other day though, depending on the size. Hopefully he heals up soon and doesn't bite anymore!